The invention relates to x-ray machines and specifically to automated setting of technic factors such as exposure time, KV (the operating voltage to be applied to the x-ray tube) and MA (the current in milliamperes to be supplied to the x-ray tube). In at least some x-ray machines it is important to select technic factors which avoid exposing the body to any unneeded radiation and yet produce a picture which is clear enough to be useful for diagnostic purposes. Typically three factors are set for a particular examination: the voltage and current (in KV and MA) which will energize the x-ray tube and the exposure time. The criteria for determining an optimum combination of KV, MA and exposure time include the distance between the focal spot (from which x-rays emanate) and the image receptor plane (e.g., the x-ray film plane), the type of examination or procedure (e.g., an examination of the pelvis, the skull, the stomach, the esophagus, etc.) and the thickness of the patient part which is to be imaged. Standard technic charts are published and used in the industry to find the KV, MA and exposure time for the given type of examination and for a given thickness of the body part to be imaged. In a typical prior art examination the body is positioned against the image receptor or body support, the x-ray technician measures the thickness of the body part to be imaged, using callipers or some other mechanical instrument, looks up the corresponding technic factors on the chart, and sets corresponding KV, MA and exposure time (or only KV and MAS--milliamperes per second) by using controls on a panel. The procedure is time consuming and, of course, prone to human error in that the technician may incorrectly look up or set the necessary technic factors, and the body may meanwhile move and thereby change the thickness of the part which is actually imaged.
In one known prior art system an ionization chamber is positioned adjacent the image plane to detect when the image receptor has received sufficient radiation for a clear image so that the x-ray beam can be turned off at the right time. However, the system is so expensive and requires so much careful calibration that it is believed to be rarely used outside large and sophisticated radiology centers, and it is believed that most users still rely on manually measuring the thickness of the body part to be imaged and manually setting the technic factor controls.
A prior invention by the same applicant, described in U.S. No. Pat. No. 4,403,337, which is hereby incorporated by reference, provided a system in which a non-contact automatic measurement is made of the thickness of the patient part to be imaged. This measurement is automatically used, together with technician-selected type of examination and type of body to be imaged, for automatic setting of an optimum combination of technic factors.
In the example described in detail in U.S. Pat. No. '337, a ranging transducer 20 was shown as mounted at the outside of a collimator box 22 which in turn is mounted on x-ray tube 10. Specifically, while the invention claimed in the U.S. Pat. No. '337 patent is not so limited, the transducer 20 is shown as fixedly mounted to a side wall of collimator box, outside the path of the x-ray beam from x-ray tube 10 to image receptor 16. While that exemplary arrangement is still believed to be operable and useful, it has been discovered since applying for said U.S. Pat. No. '337 patent that it is possible to have the ranging transducer in the path of the x-ray beam while making the ranging measurements but out of said path when the x-ray beam is on for the actual x-ray examination. It was discovered that, unexpectedly, significantly better results are obtained when the ranging measurements are taken when the transducer is in the path of the x-ray beam and, preferably but not necessarily, is aligned with the x-ray beam axis. Preferably, but not necessarily, the transducer movement and the x-ray tube energization circuits are interlocked so that the x-ray beam cannot be turned on while the ranging transducer is in its path--as a safety precaution against irradiating the body being examined with radiation modulated by passing through the ranging transducer structure. The movement of the ranging transducer into and out of the x-ray beam path, and the ranging operation, can be controlled either from the control panel (which typically is in an area shielded from x-ray exposure) or by means of manual controls at the structure to which the x-ray tube is affixed.
In an exemplary embodiment, an x-ray machine includes an x-ray source and an image receptor, such as film in a suitable holder, which has an image plane at a known (or determinable) distance from the source and is irradiated with an x-ray beam therefrom when the source is energized. The body to be x-rayed is positioned against a body support, on a table or against an upright support, such that the distance between the image plane and the body part to be imaged is fixed but the distance between that body part and the source is unknown--as it is determined by the unknown thickness of the body part to be imaged. A transducer fixed with respect to the source sends a ranging signal (e.g., a sonic or a light signal, visible or not) toward the body part to be imaged and receives the reflection of said radiation therefrom. In the example of using a sonic ranging signal a travel time derivation circuit is coupled with the transducer and derives therefrom a signal determined by the two-way travel time of the sonic signal, i.e., the time the sonic signal takes to travel from the transducer to the body part to be x-rayed and back from that body part to the transducer. The two-way travel time signal is converted to a thickness signal defining the thickness of the body part to be imaged with x-rays, and an exposure time derivation circuit derives, in part on the basis of that thickness signal and in part on the basis of technicianselected push buttons for the type of x-ray examination and for the type of body physique, a signal defining the exposure time for imaging said patient part. The type- of-examination and type-of-body-physique push buttons (or other manually operated devices) which are set by the technician automatically determine the KV and MA at which the x-ray source will be operated. The system can periodically recheck the body thickness and, if necessary, update the automatic setting of technic factors, until the technician pushes a button (or operates some other control) to initiate actual x-ray exposure at the so selected technic factors. Failsafe provisions are made against exceeding a maximum permissible exposure time which can be selected by the technician for the particular type of examination and/or body, and maximum permissible MAS (current flow per second to the x-ray tube).
The ranging transducer is mounted to move between an active position, in which it is in the path of the x-ray beam, and preferably but not necessarily aligned with its axis, and an inactive position, in which it clears that x-ray beam. Means can be provided for interlocking the transducer movement with the x-ray tube energization circuit to prevent turning on the x-ray beam while the ranging transducer is in its path.